1. Field of the Invention
The present invention relates to a process for producing a resin molded article and a mold which is used in the process. In accordance with the present invention, a resin molded article having a fine uneven structure on the surface can be produced advantageously. For example, the present invention can produce (1) a light-conducting plate used in a back light of a liquid-crystal display or the like, (2) a lens sheet such as a Fresnel lens sheet or a lenticular lens sheet used in a screen of a liquid-crystal projection TV or a projector, a condensing Fresnel lens sheet or the like, and (3) a substrate of an optical recording medium that retrieves an information of an image or the like or that optically records or retrieves the same.
2. Description of the Related Art
In general, a molded article having a fine uneven structure on the surface is produced by injection-molding a thermoplastic resin. As the molded article, a substrate of an optical recording medium is mentioned. An optical recording medium such as an optical disk has been spread worldwide since a compact disk (CD) and a laser disc (LD) began to be marketed in the early 1980s. The development and the study of a high-density optical disk in which large-capacity data such as a moving information of approximately 2 hours is digitally recorded on a clear resin molded substrate of a CD size and which is thinner than conventional CD and LD have been currently conducted. This large-capacity optical disk is generally produced by, in view of the mass-production and the cost, a method in which a clear resin is injection-molded, and pits or grooves on a surface of a stamper mounted on a mold are transferred thereon to form a substrate.
In the injection-molding step of the substrate of the optical recording medium, the molten resin is injected into the cavity of the mold, and the pressure is exerted within the cavity through the molten resin of a sprue or a runner in the pressure holding step until a gate is completely cooled and solidified, thereby transferring the shape of the mold. After the gate is solidified, the resin in the mold is cooled and solidified to obtain a molded article (substrate of optical recording medium).
When the molten resin injected into the cavity by the above-mentioned injection-molding is brought into contact with the cavity surface, this molten resin is abruptly cooled, and filled into the cavity while a cooled and solidified layer is formed. The thus-cooled and solidified layer decreases the transferability, causes occurrence of a weld mark and a cold mark that induce abnormal light emission at a portion in contact with the molten resin filled, decreases the strength at the weld mark and the cold mark, decreases qualities owing to the deformation by residual stress, deteriorates appearance due to the deformation and causes occurrence of a flow mark.
A conventional method for producing a light-conducting plate is described below. A structure of an illuminator used in a back light of a liquid-crystal display is shown in FIG. 9. As shown in FIG. 9, the illuminator comprises a light source 11 such as a cold cathode, a light-conducting plate 12 mounted such that an incident end surface 12a is situated in the vicinity of the light source 11, a diffusion sheet disposed on the surface of the light-conducting plate 12, and a reflection sheet 14 mounted opposite to the diffusion sheet 13 of the light-conducting plate 12. In the illuminator having such a structure, a light from the light source 11 is entered into the light-conducting plate 12 from the incidental end surface 11a. The light entered into the light-conducting plate 12 is transmitted in the direction opposite to the incidental end surface 12a while being reflected on the surfaces of the diffusion sheet 13 and the reflection sheet 14. During that time, a part of the light comes out of the surface of the light-conducting plate 12, passes through the diffusion sheet 13 and is released outside the illuminator as a diffused light to obtain an illumination light having a uniform brightness.
In the above-mentioned conventional illuminator, in order to obtain a uniform diffused light, a pattern having a dot-like distribution was printed on a reverse surface (surface on a reflection sheet 14 side) of a light-conducting plate, or the reverse surface was roughened or embossed, or a pattern having a prism-like distribution was given thereon.
When a light-conducting plate is produced by injection molding, a pattern having a dot-like distribution is formed on the light-conducting plate using a mold in which an uneven pattern opposite to a desired uneven pattern is formed in a predetermined region.
The cooled and solidified layer formed when the molten resin injected into the cavity by the injection molding is brought into contact with the cavity surface and abruptly cooled involves problems that the photoconductive sheet, like the substrate of the optical recording medium, decreases the transferability, causes abnormal light emission, decreases the strength, causes deformation, gives a poor appearance and causes occurrence of a flow mark.
A conventional method for producing a lens sheet is described below. When producing a lens sheet having a large area such as a Fresnel lens sheet or a lenticular lens sheet, generally a flat lens mold heated is brought into contact with a resin plate, and pressed to transfer an uneven lens surface of the lens mold on the resin plate. In this method, however, a molding cycle is long, and a productivity is not high. Therefore, a technique has been recently developed in which an ultraviolet curing resin is coated on a lens mold, and a resin plate is placed thereon, and irradiated with an ultraviolet light to form a lens.
Meanwhile, a Fresnel lens sheet or a lenticular lens sheet having a relatively small size is produced by injection-molding a synthetic resin. When a lens sheet is produced by injection molding, a lens surface is processed using a mold in which an uneven pattern opposite to an uneven pattern of a desired lens surface is formed.
The cooled and solidified layer generated when the molten resin injected into the cavity by the injection-molding is brought into contact with the cavity surface and abruptly cooled involves problems that the lens sheet, like the substrate of the optical recording medium, decreases the transferability, causes abnormal light emission, decreases the strength, causes deformation, gives a poor appearance and causes occurrence of a flow mark.
In order to suppress the generation of the cooled and solidified layer that decreases the transferability in the conventional injection molding, it is generally considered that the molding conditions are changed such that the temperature of the molten resin is increased and the injection rate is increased or the mold temperature is controlled using a cryogenic cycle temperature regulator. However, in this method, the resin is thermally deteriorated or yellowed due to the prolongation of the molding cycle and the light transmission is decreased, so that a substrate of an optical recording medium causes insufficient reading of a signal, or a molded article rendered at a high temperature in molding is forcibly released from the mold to deform the molded article or decrease the productivity. Accordingly, this method is not satisfactory.
On the other hand, in order to reduce a non-uniformity of a stamper temperature, Japanese Patent Laid-Open No. 26,616/1991 discloses a technique in which a stamper is closely adhered to a mold using a magnet, and Japanese Patent Laid-Open No. 224,921/1992 discloses a technique in which a stamper is closely adhered to a mold using a viscous thin film. In these techniques, the overall transfer surface of the stamper in the mold can uniformly be cooled, making it possible to provide a uniform transferability of a substrate of an optical recording medium. However, the cooled and solidified layer of the resin filled cannot be decreased, making it impossible to improve the transferability. Further, Japanese Patent Laid-Open No. 180,541/1987 discloses a technique in which a material having a thermal conductivity of 7.times.10.sup.-2 cal/cm.multidot.sec.multidot..degree. C. or less is coated on a reverse surface of a stamper used in the injection molding of a compact disk or the like, and Japanese Patent Laid-Open No. 178,774/1995 discloses that a heat-insulating mold insert is mounted on a reverse surface of a stamper to retard initial cooling of a thermoplastic material during molding. However, the techniques described in these documents provide an insufficient consideration on a relationship between properties of a resin used in the injection molding and a heat capacity of a mold including a stamper. Accordingly, when the techniques described in these documents are applied to the production of a recent optical disk having a higher density than CD or LD and having finer pits or grooves, for example, a heat supplied from the molten resin to the stamper is rapidly escaped into a mold, whereby occurrence of the cooled and solidified layer cannot be controlled and no sufficient transferability can be provided.
The process for producing the resin molded article in the present invention has been achieved to solve the above-mentioned problems. The present invention is to provide a process in which formation of a cold and solidified layer on a surface of a molten resin injected into a cavity is minimized to improve a transferability and to decrease occurrence of a weld mark, a cold resin mark and a flow mark, and a high productivity is realized. Further, the mold of the present invention is used to produce the resin molded article of the present invention.